Many microelectromechanical system (MEMS) and microelectronic devices can be damaged if exposed to high temperatures during some fabrication steps, such as packaging steps. For example, the temperature of a die bonding process may be a significant concern when selecting a method to attach a MEMS component to a package substrate or to integrate a MEMS component with a microelectronic component.
A MEMS component can include a glass isolation layer that is attached to an alumina package component via one of a variety of die attach materials. A gold-tin brazing material is commonly used in microelectronic packaging, however, the brazing temperature will be too high for some types of components.
Other bonding techniques that can be used in MEMS or microelectronic devices include fusion bonding (for example, for silicon-to-silicon bonding), gold eutectic bonding (for example, for silicon-to-substrate bonding), and anodic bonding (for example, for silicon-to-glass; anodic bonding is also known as Field Assisted Bonding and Electrostatic Bonding.) These bonding processes typically require application of an elevated temperature. For example, a conventional silicon-to-silicon fusion bonding process utilizes a bonding temperature of above 1,000° C., and a conventional silicon-to-glass anodic bonding process utilizes a bonding temperature in a range of approximately 200° C. to 500° C., often approximately 450° C.
Many conventional bonding processes are not applicable when fabricating or packaging devices that involve temperature-sensitive components. Many proposed solutions to this bonding problem are either complex or poorly adaptable to large scale production.
Some MEMS devices include silicon carbide (SiC) components. For example, some devices include a portion of a single-crystal SiC wafer on an insulator layer. Such a structure is useful, for example, for microwave applications. The SiC-insulator structure can be formed by anodically bonding a flat single-crystal SiC wafer to a high temperature-resistance glass. The single-crystal SiC wafer used for anodic bonding typically has a root-mean-square surface roughness of less than 2.0 nm (i.e., less than 0.002 μm.) Conventional single-crystal SiC bonding is performed at temperatures as high as approximately 560° C. and voltages between 800-1000V.
Single-crystal SiC-on-glass substrates are used, for example, to grow epitaxial layers of GaN. SiC provides a suitable crystallographic match for GaN. Hence, GaN can be epitaxially grown on a highly polished, appropriately oriented surface of a single crystal of SiC. The GaN in turn is used, for example, to form blue light-emitting diodes and lasers.
In addition to requiring elevated temperature processing, some bonding materials, such as a gold-tin braze material, can creep after bond formation. Creep can cause a bond to be unstable and to fail. Unfortunately, substrates based on anodically bonded single-crystalline SiC generally are not of use in MEMS applications.